To convert an image, various types of image conversion processes are being employed. However, when the wavelength component of image data is modulated in an image conversion process, there may occur so-called “ringing,” where a streak appears around an edge.
Patent Literature 1 discloses an image sharpening apparatus that aims to suppress emphasis of ringing and to sharpen a blurred image and includes a storage unit that stores an input image and a temporarily sharpened image having the pixel value of the input image as an initial value, a determination unit that determines whether each pixel of the input image belongs to an edge region, a range calculator that calculates the upper and lower limits of pixel values in a limited range from the distribution of the values of pixels near an edge pixel determined to belong to the edge region in the input image or the distribution of the values of pixels near a pixel in the temporarily sharpened image corresponding to the edge pixel, and a converter that (a) calculates a trial pixel value which is the sum of values obtained by multiplying the values of the pixels near the corresponding pixel by weighting factors corresponding to the distances from the corresponding pixel to the near pixels, (b1) obtains the converted values of the pixels near the corresponding pixel so that the difference between the value of the edge pixel and a trial pixel value calculated from the converted values of the pixels near the corresponding pixel becomes smaller than the difference between the value of the edge pixel and the trial pixel value, and (b2) writes, to the pixel values of the temporarily sharpened image, pixel values obtained by correcting the converted pixel values so that the converted pixel values fall within the limited range.